In renal failure, the elevation in blood urea counteracts the fall in serum osmolality due to hyponatremia. Here, the plasma water sodium concentration and plasma osmolality are unchanged, but the measured sodium concentration in the total plasma volume is reduced since the specimen contains less plasma water. Plasma water fraction falls below 80 percent in cases with marked hyperlipidemia (triglycerides >1500 mg/dL) or hyperproteinemia (protein >10 g/dL). In normal subjects, the plasma water is 93 percent of the plasma volume, fats and proteins account for the remaining 7 percent. Pseudo (normo-osmolal) or isotonic hyponatremia is due to presence of hypertriglyceridemia or increase in plasma proteins in conditions such as multiple myeloma. The serum osmolality (S Osm) can be calculated by the concentration in millimoles per liter of the major serum solutes according to the following equation: Sosm (mmol/kg) = (2 × serum ) + (serum /18) + (blood urea nitrogen/2.8). Normal serum osmolality is 280-295 mosm/kg. Hyponatremia in the elderly may manifest with frequent falls and gait disturbances. Sometimes, subtle neurologic abnormalities may be present when the serum sodium is between 120 and 130 meq/L. Mild hyponatremia is characterized by gastrointestinal tract symptoms nausea, vomiting, loss of appetite. Hence in chronic hyponatremia patients may appear asymptomatic. This is a protective mechanism that reduces the degree of cerebral edema it begins on the first day and is complete within several days. Brain adapts itself to hyponatremia by generation of idiogenic osmoles. The serum sodium concentration is usually above 120meq/L. Acute hyponatremia is characterized by onset of symptoms 48 h should be considered “chronic.” Most patients have chronic hyponatremia.
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